Ultrasonic scanning systems are currently utilized in a variety of medical and industrial applications to non-invasively and non-destructively obtain images of organs and other body parts and of mechanical structures at depths which are not visible from the surface.
One problem with most existing ultrasonic scanning systems is that the transducer is typically focused to a predetermined depth, both for transmit and receive. This means that the ultrasonic beam is relatively wide both in the azimuth direction (which is parallel to the direction in which the transducer is typically scanned) and in the elevation direction (which is at right angles to the direction of scan) at points which are less deep than the focal point and, in particular, at points at greater depths than the focal point. Where the area of interest is only at a single depth, this may not be a problem. However, the area of interest with an ultrasonic scan typically extends over a substantial depth range. Therefore, the beam is typically focused somewhere near the midpoint of this range and is thus out of focus by varying degrees through much of the scan.
Various techniques have been utilized in the past for overcoming this problem. However, most of these techniques have continued to utilize a single transmit aperture having a single focal point for transmission and have varied aperture, generally in the azimuth direction, on receive in an attempt to maintain a more uniform f number and to obtain a variable focus. U.S. Pat. No. 4,359,767 and 4,670,683 teach the use of similar techniques to control receive focus in the elevation direction.
However, these techniques address only half of the problem in that, even though resolution may be enhanced by providing a variable receive focus as echoes from greater depths are being received, resolution is still degraded by the fact that the transmit beam is out of focus through much of the depth range of the scan. While a commercial system was recently introduced which provided multiple transmit apertures at a given steering angle in the azimuth direction, with line splicing to permit utilization of data for the appropriate transmit scan line at different depths, systems which provide the proper aperture and focus either for transmit receive in the elevation direction or for transmit and receive in both the elevation and azimuth directions do not exist in the prior art. Further, since transmitting multiple scan lines at a given steering angle results in significant reductions in frame rate, which may be unacceptable when scanning moving organs such as the heart, the prior art has not dealt with the problems of enhancing frame rate to compensate for the use of multiple transmit apertures. The availability of multiple scan lines at a particular steering angle also presents opportunities for further resolution enhancement which have not been addressed in the prior art.
A need therefore exists for an enhanced ultrasonic scanning system which provides for multiple scanning apertures in the elevation direction and/or in both the elevation and azimuth direction to provide proper transmit and receive focus at varying scan depths and thus enhanced image resolution. When doing this, it is also desirable to provide means for reducing the degradation in frame rate caused by multiple scanning apertures and to utilize the multiple scan lines being transmitted from multiple apertures at a given steering angle to obtain still greater resolution enhancement.